This invention relates to thermal ink jet printing and, more particularly, to thermal ink jet printing of images which may be read by magnetic image character recognition (MICR) devices.
A thermal ink jet printer has at least one printhead in which thermal energy pulses are used to produce vapor bubbles in ink-filled channels and so cause droplets of ink to be expelled from the channel orifices towards a recording medium. The thermal energy pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable by current pulses to heat and vaporize ink in the channels. As a vapor bubble grows in any one of the channels, ink bulges from the channel orifice until the current pulse has ceased and the bubble begins to collapse. At that stage, the ink within the channel retracts and separates from the bulging ink which forms a droplet moving in a direction away from the channel and towards the recording medium. The channel is then re-filled by capillary action, which in turn draws ink from a supply container. Some arrangement is usually provided to clean the channel orifices periodically while the printhead is in use and to close-off the orifices when the printhead is idle to prevent the ink in the printhead from drying out.
One form of thermal ink jet printer is described in U.S. Pat. Nos. 4,638,337 to Torpey et al., and 4,571,599 to Rezanka. These printers are each of the carriage type and have a plurality of printheads, each with its own ink supply cartridge, mounted on a reciprocating carriage. The channel orifices in each printhead are aligned perpendicular to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped, perpendicular to the line of carriage movement, by a distance equal to the width of the printed swath and the carriage is then moved in the reverse direction to print another swath of information. As an alternative to providing each printhead with its own ink cartridge, the printheads can be supplied with ink from one or more supply tanks which need not be mounted on the carriage.
In another form of thermal ink jet printer, several printheads are accurately juxtapositioned to form a pagewidth array which remains stationary while the recording medium is moved at a constant speed in a direction perpendicular to the length of the array. Printers of that type are described in U.S. Pat. Nos. 4,463,359 to Ayata et al (see FIGS. 17 and 20) and 4,829,324 to Drake et al. In these particular printers, the printheads are mounted on a common metal plate which functions as a heat sink, for efficient dissipation of heat generated when the printer is in operation.
Images for MICR must contain magnetic material which by its remanence, coercive force, saturation magnetization and mass per unit area, assures reliable character recognition, preferably in existing reading equipment. When liquid ink is used for printing MICR images, this ink contains magnetic pigment and this dispersion must be stable with sufficiently long shelf life. The economy, performance and reliability of MICR printing is improved with the increased magnetic efficiency of the marking material, in terms of achieving the desired signal with less mass of the magnetic pigment per unit area.
U.S. Pat. No. 4,103,306 to Clapp discloses a nonimpact printer having a support for magnetic ink particles loosely distributed on its surface in mutually spaced aggregates of irregular height. The patent describes a process whereby an electrical field of short duration, established in a print position between the particles and a shaped print electrode, charges the particles and attracts them to an intervening recipient sheet so that a printed image is rendered which is more uniform by the magnetic re-orientation of the aggregates of greater height before printing.
U.S. Pat. No. 4,197,563 to Michaud discloses a method and device for orientating and fixing in a determined direction magnetic particles contained in a polymerizable ink. The patent states that the method of orientating and fixing comprises spreading magnetic, polymerizable ink on a support; conveying the support toward a zone subjected to a magnetic field; depositing the ink on the support and polymerizing by means of an ultraviolet lamp; and permanently fixing the image with the magnetic ink particles in proper orientation.
U.S. Pat. No. 3,972,715 to Olumura, discloses a particle orientation imaging system wherein an imaging member comprising a layer of softenable, electrically insulating material containing a dispersion of randomly orientated electrically photosensitive particles is imaged. The patent states that the imaging process comprises applying a uniform electric field across the member; imagewise exposing the member to activating electromagnetic radiation; and developing the member by decreasing the resistance of the softenable layer to reorientation of the magnetic particles sufficiently to allow such a reorientation.
U.S. Pat. Nos. 3,845,499 and 3,683,382, both to Ballinger, disclose an apparatus and a recording medium responsive to force fields for orientating magnetic particles having fixed and varying magnetic field components and a method of reproducing signals on the medium. The patent discloses a recording medium which is responsive to a magnetic or electrostatic field for recording an input signal thereon having a continuous web of encapsulated or entrapped material of highly reflective flakes. Exposure to a magnetic or an electrostatic field reorients the pre-orientated flakes to provide a contrast between the exposed and unexposed portions.